The present invention relates to a method for removing the hard coat film from plastic lenses coated with a hard coat film.
Being different from glass lenses, plastic lenses are characterized in that they are lightweight, hardly cracked and easily colored. For such plastic lenses, acrylic resins, polycarbonate resins and ADC resins have heretofore been used. Recently used are high-refractive-index urethane resins, and those having a refractive index of larger than 1.6 have been commercialized in the market. Though having various advantages such as those mentioned above, plastic lenses are inferior to glass lenses in point of scratch resistance and heat resistance. To improve the scratch resistance of plastic lenses, proposed was a method of applying a protective coat onto the surface of the lenses, for which were proposed a numerous number of coating compositions of different types. For example, known are coating compositions consisting essentially of an organic silicon compound or its hydrolysate, such as those disclosed in Japanese Patent Application Laid-Open (JP-A) Sho-52-11261; and coating compositions containing a colloidal dispersion of silica sol, such as those in JP-A Sho-53-111336, the latter being for further improving the scratch resistance of plastic lenses coated therewith. Methods of applying such coating compositions onto plastic lenses have already been put into practical and industrial use.
For lens materials, ADC resin such as typically PPG's CR-39 has heretofore been widely used in producing plastic lenses for a long period of time. However, lenses of ADC resin have a refractive index of about 1.50, which is lower than that of glass lenses. Therefore, using the resin is problematic especially for powerful minus lenses for short-sightedness in that the peripheral edge of the lenses is to be thick. Those lenses of the resin look unattractive, and users do not like them. Lens materials having a higher refractive index than CR-39 are known. For example, JP-A Sho-63-46213, Hei-2-270859, Hei-2-275901 and Hei-3-56525 disclose sulfur-containing polyurethane resins for plastic lenses, and high-refractive index polyurethane lenses having a refractive index of higher than 1.6 have been put into practical use. Coating compositions such as those mentioned above are applied to those lenses. However, the coated lenses are defective in that they have interference fringes and look unattractive. It is said that such interference fringes appear due to the difference between the refractive index of the lens material and that of the overlying coat. A single-layered or multi-layered, non-glare layer of an inorganic compound is often formed over the coat. If the non-glare layer is formed, however, the color reflected on the layer is seen too greatly, resulting in that the lenses look more unattractive. In order to overcome those drawbacks, various studied have heretofore been made for increasing the refractive index of the coat of plastic lenses. For example, JP-A Hei-2-245078, Hei-2-261827, Hei-4-126784 and Hei-7-325201 disclose hard coat compositions containing high-refractive-index inorganic grains.
A high-refractive-index hard coat basically comprises an organic silicon compound and/or its hydrolysate, and inorganic grains, and its refractive index shall be slightly lower than or comparable to that of high-refractive-index base lenses to evade interference fringes. The organic silicon compound and its partial hydrolysate may be selected from alkylalkoxysilanes and/or ordinary silane-coupling compounds having reactive organic groups of epoxy group, vinyl group, (meth)acryl group, amino group, etc. Of those, especially preferred are epoxy-containing compounds. The inorganic grains may be of oxides such as silica, iron oxide, titanium oxide, cerium oxide, zirconium oxide, antimony oxide, zinc oxide or tin oxide, their mixtures or composite oxides. Of those, preferably used are titanium oxide, cerium oxide, iron oxide and zirconium oxide to form high-refractive-index coats applicable to high-refractive-index lens materials. Silica is said to have the activity of increasing the dispersion stability of other inorganic grains, and is often used as an aid in high-refractive-index coats comprising other inorganic grains. The inorganic grains constituting the coats may have a mean grain size of from 1 to 300 nm, but especially preferably from 5 to 100 nm. Apart from those components noted above, various additives such as catalyst, UV absorbent, film-forming resin or its precursor and leveling agent may be added to the coats to thereby improve the characteristics of the coats and even those of the coating compositions.
In the industrial process of producing plastic lenses, the reduction in the production yield due to processing failure is a critical problem as causing the increase in the production costs. Specifically, processing failure in hard coat formation or processing failure in non-glare coat formation which is often effected after the hard coat formation is a bar to the reduction in the production costs. Naturally, any processing failure shall be evaded so as to increase the production yield. In fact, however, some processing failure is inevitable. The processing failure in hard coat formation includes, for example, presence of impurities on or in the coat, coating unevenness, and whitening of the coat, and the failed products could not be commercial ones. Recycling those failed products is important for reducing the production costs and even for effective use of natural resources.
To remove the coat from plastic lenses, known are a method of physically peeling or cutting off the coat, and a method of releasing or dissolving the coat with some chemicals. The latter chemical method is preferred in order to reuse the coat-removed lens bases. For the removing method with chemicals, for example, it is written in Industrial Materials, Vol. 30, No. 8 that silicone hard coat resin is dissolved out when dipped in an aqueous solution of 10% sodium hydroxide for 24 hours. Japanese Patent Publication (JP-B) Hei-2-36309 discloses a method of recycling plastic lenses, comprising dipping plastic lenses coated with an organopolysiloxane hard coat in an aqueous solution of an alkali metal hydroxide to remove the coat; and JP-B Hei-3-5227 discloses a method of recycling plastic lenses, comprising dipping plastic lenses coated with a hard coat consisting essentially of an organic silicon compound and a curable organic compound, in an alkaline aqueous solution containing an aqueous solution of surfactant to remove the coat. In those methods, the hard coat of plastic lenses is corroded and dissolved to be thereby removed from the lenses. In those, however, inorganic substances are difficult to remove and, even if removed, they will again adhere to the lenses. For this, surfactant is added to the dipping solution to evade the re-adhesion. The methods are extremely effective in removing hard coats consisting essentially of an organopolysiloxane compound and formed on low-refractive-index lenses of CR-39.
However, high-refractive-index hard coats are more difficult to remove than low-refractive index ones. Therefore, if plastic lenses coated with such a high-refractive-index hard coat are desired to be removed in the method of dipping the coated lenses in an aqueous solution of an alkali hydroxide, and if the coated lenses are treated in the solution under the condition, temperature, time and alkali hydroxide concentration, for completely removing their hard coat, the urethane lens base is greatly influenced by the treatment resulting in that the surface of the base is partly corroded and roughened. The dipping methods are especially unfavorable for the removal of high-refractive-index hard coats from sulfur-containing polyurethane lenses, as being basically problematic in that some coat residue still remains on the lens base even after the treatment, that the lens base is poorly resistant to the aqueous solution of an alkali metal hydroxide and is therefore often roughened, and that the base lens treated could not be effectively recycled.